If you are willing to have a leather V-belt spliced to order, this
might be your best bet but, short of that, you should realize that not
all belts are created equal.

An automotive belt has a different cross section and stiffness than a
fractional horsepower belt and therefore is likely to be much noisier
in a piano.  Also different manufacturers pay attention differently to
details.  I have found that belts by Dayco are much quieter in a piano
as they seem to be more uniform in cross section and have a smoother
back side, which makes a lot of difference in a Duo-Art pump with a belt
tensioner.

If you are willing to be unconventional and have a flaming orange or
green belt in your piano, the best belts are the heat spliced urethane
belts.  You can buy the bulk material and splice them with or without
the proper tools and they will be the quietest belts you can buy and
can be made to any length.

It takes a little practice to splice them without the recommended
tools, but it can be done.  You can make an alignment jig out of almost
anything and heat a metal rod to melt the belting to fusion
temperature.  Then carefully trim off the slop and you have a very nice
belt after about the fifth or sixth attempt.  They can also be spliced
professionally and will be perfect the first time.

Spencer Chase
http://nt.mcn.org/spencer/